English translation
Sun Tzu said: In former times those skilled in war first made themselves invincible, and then waited for the enemy to be vulnerable. "Invincible" depends on oneself; "vulnerable" depends on the enemy. Therefore the skillful warrior can make himself invincible but cannot make the enemy necessarily vulnerable. Thus: victory can be recognized, but it cannot be forced.
Chinese source
孫子曰：昔之善戰者，先為不可勝，以待敵之可勝。不可勝在己，可勝在敵。故善戰者，能為不可勝，不能使敵之必可勝。故曰：勝可知，而不可為。
Being invincible is a matter of defense; vulnerability to defeat invites attack. To defend is to appear lacking; to attack is to possess surplus. The good defender conceals himself beneath the ninefold earth; the good attacker moves above the nine heavens—thus they can preserve themselves and secure complete victory.
Chinese source
不可勝者，守也；可勝者，攻也。守則不足，攻則有餘。善守者，藏於九地之下；善攻者，動於九天之上，故能自保而全勝也。
To win in ways that everyone can see is not the highest skill; to win in battle and have the world call it skill is not the highest excellence. Lifting a single autumn hair is not proof of great strength; seeing the sun and moon is not proof of superior sight; hearing thunder is not proof of keen hearing.
Chinese source
見勝不過衆人之所知，非善之善者也；戰勝而天下曰善，非善之善者也。故舉秋毫不為多力，見日月不為明目，聞雷霆不為聰耳。
What ancient writers called being skilled at war was winning where victory was easy. Therefore the victories of the skilled carry no name for cleverness and no claim of heroic deed. Their victories do not go wrong. "Not going wrong" means that where they place themselves they will certainly win; they win over foes already doomed. Thus the skilled commander stands on ground where he cannot be defeated and does not miss the enemy's collapse. Therefore a victorious army first secures victory and then seeks battle; a defeated army first engages in battle and then seeks victory. The effective commander cultivates the Way and preserves the laws, and so can direct matters of victory and defeat.
Chinese source
古之所謂善戰者，勝於易勝者也。故善戰者之勝也，無智名，無勇功。故其戰勝不忒。不忒者，其所措必勝，勝已敗者也。故善戰者，立於不敗之地，而不失敵之敗也。是故勝兵先勝而後求戰，敗兵先戰而後求勝。善用兵者，修道而保法，故能為勝敗之政。
On military method: first measurement, second estimation, third calculation, fourth comparison, fifth victory. Terrain gives rise to measurement; measurement gives rise to estimation; estimation gives rise to calculation; calculation gives rise to comparison; comparison gives rise to victory. Thus a victorious army weighs ounces by the pound, while a defeated army weighs the pound by the ounce. The fighting of the victorious is like releasing accumulated water down a thousand-foot gorge—that is its form.
Chinese source
兵法：一曰度，二曰量，三曰數，四曰稱，五曰勝。地生度，度生量，量生數，數生稱，稱生勝。故勝兵若以鎰稱銖，敗兵若以銖稱鎰。勝者之戰民也，若決積水於千仞之溪者，形也。